This invention relates to control systems for controlling the speed of DC motors. More particularly, this invention relates to control systems for battery-powered electric motors used in lawnmowers, snowblowers, and other lawn and garden equipment.
Due to concerns regarding air pollution, electric lawnmower motors are becoming increasingly popular as a replacement for small, internal combustion engines used on lawnmowers, snowblowers and the like. In particular, battery-powered motors are desirable instead of AC line-powered units due to the inherent operating limitations and safety problems when electric power cords are used.
A major disadvantage of typical prior art battery-powered motors is that the motor speed tends to be reduced or "droop" as the battery becomes discharged. When the motor speed drops below a certain level, the lawnmower output becomes too low for the lawnmower blade to effectively cut the grass.
Another problem with battery-powered lawnmowers occurs when the blade attempts to cut wet or heavy grass. The torque output of the motor is often insufficient to overcome the resistance of heavy or wet grass, thereby preventing the grass from being cut.
When the load increases as, for example, when the blade encounters heavy or wet grass, the current to the motor is increased in some prior art devices to compensate for the increased load. This increase in current may quickly discharge the battery unless it is controlled.
Prior art control systems exist for controlling the speed of electric motors. However, these control systems typically are complicated and expensive since they require many discrete components. Also, these prior art systems often are not capable of providing increased torque for short periods of time and they typically do not sufficiently protect the battery from unnecessary, premature discharge.